Power converters are electrical devices that are used to convert one voltage to another voltage. In most instances, they take a variable voltage and convert that voltage to a fixed voltage. In the case of the topologies discussed herein, the converters do this by varying the duty cycle of the power switches. If the input voltage decreases, the control circuit increases the ON time of the transistor switches so that the output remains at the desired voltage.
One of the topologies used in converting power from one voltage to another is the forward converter. In this topology, a primary side voltage is applied across a winding of a transformer and through that a voltage is induced in the secondary of the transformer. The voltage across the primary is applied and removed by cycling switches. The alternating application and removal of the primary voltage causes a DC voltage to appear on the secondary of the transformer. This voltage is rectified, filtered, and applied to a load across the output. When the switches are on, the voltage VIN across the primary of the transformer leads to a build-up of magnetizing current within the transformer. To prevent failure of the transformer, this magnetizing current must be reset to zero during the OFF portion of the cycle.
Another topology used in power conversion is the fly-back converter. In this topology, when a voltage is applied across a primary winding of a transformer, the primary current and magnetic flux increases, storing energy in the transformer, but no voltage is able to flow in the secondary winding due to a reverse-biased diode. When the voltage is removed from the primary winding, the primary current and magnetic flux drops. The secondary voltage is reversed, forward-biasing the diode and allowing current to flow from the transformer.